1. Field of the Invention
This invention relates generally to echo cancellation. More particularly, it relates to a configurable echo canceller which adapts between audio echo cancellation and/or hybrid echo cancellation based on the particular application.
2. Background of Related Art
An echo canceller is an electrical device which removes interfering information from a signal caused by an electrical or acoustical echo of the signal. Echo cancellation has many uses, but is well known for its use in telephone systems to remove echoes during a conversation on a telephone line.
Electrical echoes are most typically caused in customer premises telephone equipment (e.g. a telephone) when a signal passes through a hybrid (otherwise known as a telephone line interface (TLI)) to the telephone line. A hybrid provides isolation between the telephone and the telephone line. The better the hybrid, i.e., the better the isolation, the larger the reflection or echo of the signal back into the telephone. A conventional device used at the customer premises telephone equipment to eliminate the effect of reflections or echoes caused by the hybrid are generically termed hybrid echo cancellers (HECs).
Other reflections of a telephone signal causing audible echoes in the conversation are caused by the physical environment in which the telephone sits. For instance, acoustical reflections are caused from the speaker back to a microphone in the telephone when sounds output from the speaker are picked up by the microphone. This most frequently occurs when the output sound bounces off the ceiling, walls and other structures in the room. Acoustical reflections typically cause uncomfortable feedback such as squealing in the signal output from the speaker.
Different acoustical reflections or echoes may also be caused at different times, depending on the distance between the speaker, the particular reflecting surface (e.g., the ceiling or any of the four walls), and the return path to the microphone. For instance, an acoustical echo would be caused on a telephone line a fixed amount of time after the speech originated. This fixed amount of time is based on the time it takes sound to travel from the speaker, to the reflecting surface, and then to the microphone. Not all rooms are the same size, nor is a telephone placed in the same spot in all rooms. Thus, because of the uncertainty as to the particular dimensions of a room and the proximity of the telephone to the walls and/or other structures in the room, it is difficult to know beforehand the specific needs of an acoustical echo canceller. Higher end acoustical echo cancellers adaptively train themselves to the particular acoustical echoes experienced by a telephone, while lower end acoustical echo cancellers have predetermined tap values set on the presumption of certain average or median conditions, e.g., 6 feet from the speaker and microphone to the ceiling (assuming that the customer premises telephone equipment is placed on a table), and, e.g., 10 feet from the speaker and microphone to the surrounding wall surfaces. Acoustical echo cancellers are most prevalent in speakerphone type applications.
Initially, echo cancellers were analog devices which sampled a pure main signal at a point before it was affected by a reflection or echo, delayed the sampled pure signal, and subtracted the delayed pure signal from the main signal to output a signal corresponding to the pure main signal with the effects of any reflections or echoes removed. Thus, ideally, the delayed pure signal cancels out the reflected signal picked up by the microphone and reintroduced into the main signal.
More recently, particularly as digital signal processors (DSPs) and other high performance processors became cost effective, software algorithms have been developed to perform an acoustical echo cancellation or a hybrid echo cancellation on a digitized signal. Conventional acoustical echo cancellers suppress audio picked up by a microphone which was output by a speaker on the same telephone. These conventional AECs are typically adaptive, speech trained and monitored and adjusted as necessary for optimal performance.
Conventional customer premises telephone equipment includes an acoustic echo canceller and/or a hybrid echo canceller as appropriate for the application. For instance, FIG. 6 shows the relevant portion of a cordless telephone including a cordless handset 602 and a base unit 604. The cordless handset 602 typically includes a microphone and a speaker to allow a telephone conversation to take place, and an acoustical echo canceller 606 to cancel any acoustical echoes caused in the telephone signal. There is typically no hybrid echo canceller in the cordless handset 602 because there conventionally are no hybrids in the cordless handset 602.
On the other hand, the base unit 604 interfaces to a telephone line from a central office via a telephone line interface, i.e., a hybrid. Accordingly, a hybrid echo canceller 608 is typically included in the base unit 604 of a cordless telephone to remove unwanted signal disruptions and noise caused by reflections of the main signal caused by the hybrid in the base unit 604. However, because the base unit 604 does not typically include a speaker and a microphone, there is typically no AEC in the base unit 604.
Accordingly, a designer conventionally picks and chooses an AEC or an HEC based on a particular application, and manufacturers provide either an AEC or an HEC for each particular application. For instance, in a cordless handset 602, a particular AEC might be chosen for use while in the base unit 604, a particular HEC might be chosen. This causes the need to manufacture such a cordless telephone with two separate echo cancellation parts, with associated inventory and costing requirements.
In the event of a combined requirement for both an AEC and an HEC within the same unit, manufacturers will typically customize a combined HEC and AEC for use with the particular application.
Thus, many different types of echo cancelers must be manufactured, inventoried and maintained, e.g., AECs, HECs, and AEC/HEC combinations. Unfortunately, the use of many different types of echo cancelers complicates manufacturing, maintenance, and overall cost.
There is thus a need for an echo canceller which is flexible enough to be used in various echo cancellation roles, including acoustical and/or hybrid echo cancellation roles, to reduce the need for the many different conventional types of echo cancelers.